1. Field of the Invention
The present invention relates to a telephoto zoom lens system of a four-lens-group arrangement.
2. Description of the Prior Art
An well-known conventional telephoto zoom lens system having four lens groups is typically constituted by the following four lens groups:
a first lens group which has a positive refractive power (hereinafter, a positive first lens group), which remains stationary upon zooming, and by which focusing is performed;
a second lens group which has a negative refractive power (hereinafter, a negative second lens group), which has a zooming function, and which is arranged to be movable upon zooming;
a third lens group which has a positive refractive power (hereinafter, a positive third lens group), and which is arranged to be movable for correcting a focal-point shift; and
a fourth lens group which has a positive refractive power (hereinafter, a positive fourth lens group), and which is arranged to be stationary as a master lens group for image forming.
In such a conventional telephoto zoom lens system of the four-lens-group arrangement, the overall length thereof is long, and the diameter of the most object-side lens (the positive first lens group) is large.
In Japanese Unexamined Patent Publication No. H09-133865, a zoom lens system of a five-lens-group arrangement is proposed; however, the mechanical structure thereof is complicated. Furthermore, since focusing is performed by the first lens group, such a zoom lens system is not suitable for a zoom lens system having an automatic focusing (AF) function.
In Japanese Unexamined Patent Publication No. H06-51202, an internal-focusing zoom lens system of a four-lens-group arrangement is proposed. In this four-lens-group zoom lens system, only a rear sub-lens group of the first lens group is arranged to move for focusing. Such a zoom lens system is advantageous for attaining a fixed F-number and a larger aperture, however, is disadvantageous for achieving miniaturization. Furthermore, the refractive power of the focusing lens group is small, so that the minimum photographing distance cannot be further made shorter. If an attempt is made to shorten the minimum photographing distance, the distance between the front sub-lens group and the rear sub-lens group (focusing lens group) of the first lens group has to be made longer. Consequently, the overall length of the zoom lens system becomes further longer, and the diameter of the first lens group becomes larger.
Japanese Unexamined Patent Publication No. H03-228008 proposes a zoom lens system as follows:
the zoom lens system is constituted by a positive first lens group, a negative second lens group, a positive third lens group, and a negative fourth lens group, in this order from the object;
focusing is performed by the negative second lens group; and
upon zooming from the short focal length extremity to the long focal length extremity, the positive first lens group, the negative second lens group and the positive third lens group move toward the object, and the negative fourth lens group remains stationary.
However, in such a zoom lens system proposed in Japanese Unexamined Patent Publication No. H03-228008, the F-number increases upon zooming from the short focal length extremity to the long focal length extremity.
Japanese Unexamined Patent Publication No. 2003-241097 proposes a zoom lens system as follows:
the zoom lens system having a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in this order from the object;
focusing is performed by the negative second lens group; and
all the lens groups move toward the object upon zooming from the short focal length extremity to the long focal length extremity.
The above zoom lens system is advantageous for achieving miniaturization; however, the F-number at the long focal length extremity is larger.